Password based protection schemes for credit cards or other personal identification cards are well-known in the prior art. Such cards typically include a memory comprising a magnetic tape or other storage media affixed to the card. They may also include a data processing capability in the form of a microprocessor and an associated control program. In operation, a card issuer initially stores in the memory a personal identification number, i.e., a secret password, as well as a value representing a maximum dollar amount To effect a transaction, the card is placed in a terminal and the user is required to input his or her password If the terminal verifies a match between the user-inputted password and the password stored on the card, the transaction is allowed to proceed. The value of the transaction is then subtracted from the value remaining on the card, and the resulting value represents the available user credit.
Techniques have also been described in the prior art for protecting against the illegitimate issuance of credit cards such as the type described above. In U.S. Pat. No. 4,453,074 to Weinstein, each such card has stored therein a code which is the encryption of a concatenation of a user's secret password and a common reference text. The encryption is derived in an initialization terminal through the use of a private key associated with the public-key of a public-key cryptosystem key pair. In operation, a cardholder presents his or her card to a transaction terminal. The terminal decrypts the stored code on the card in accordance with the public-key of the public-key cryptosystem pair. A transaction is effected only if the stored code decrypts into the user password, inputted on a keyboard by the cardholder, and the common reference text.
While the method described in the Weinstein patent provides an adequate protection scheme for preventing the fraudulent issuance of credit cards, this scheme requires each user to have a secret or "private" password which must be memorized and inputted into the transaction terminal. Weinstein also requires additional circuitry for concatenting the user's secret password with the common reference text. This latter requirement, while purportedly required to insure the integrity of the protection scheme, increases the complexity and the cost of the system.
It would therefore be desirable to provide an improved method for issuing personal identification cards using a public-key cryptosystem or other "proof of legitimacy" in which a "secret" password need not be memorized by the authorized user or concatenated with a common reference text to maintain the system security.